In recent years, client server systems which connect point-to-point between a client and a server using an Ethernet transmission line are increasing in number. In such a system generally a large amount of data traffic flows on the transmission line. It is therefore required to employ redundant transmission line configuration capable of switching over the lines within a short time.
In FIG. 1, there is shown an example of a block diagram of a conventional client server system having a redundant system configuration of an Ethernet transmission line. A client system 1 is connected to duplicated Ethernet transmission lines 5a, 5b through line controllers 34a, 34b. 
Further, client system 1 is provided with Ethernet controllers 33a, 33b each corresponding to a transmission line. The identical Ethernet controller 33a or 33b controls both a control frame directed to a CPU 31 and a data frame directed to an input/output controller 36 being connected to an input/output unit such as a disk unit 4. Client system 1 further includes a memory 32 for buffering data.
Meanwhile, a server system 2 is provided with equipment controller 20, a plurality of Ethernet interface controllers 30a, 30b. Equipment controller 20 further includes CPU 21, memory 22, Ethernet controller 23, network processor 25 and switch 26.
Switch 26 is set by network processor 25 so as to select Ethernet interface controllers 30a, 30b for connecting to an appropriate route.
In the example shown in FIG. 1, interface controller 30a is connected to client system 1, and interface controller 30b is connected to a terminal 6 through an IP (Internet protocol) network 3. Moreover, interface controllers 30a and 30b are mutually connected through switch 26. Thus terminal 6 can access dick unit 4 through server system 2 functioning as a router and client system 1 functioning as a file server.
Each Ethernet interface controllers 30a, 30b is provided with line controllers 24a, 24b being connected to the duplicated Ethernet transmission line, a selector 28 for selectively connecting either of line controllers 24a, 24b, a network processor 29 for controlling selector 28, and a memory 27.
In FIG. 1, CPU 31 in client system 1 distinguishes the aforementioned data frame from the control frame using data higher than the MAC (media access control) layer. Control frames are transmitted and received between CPU 31 and CPU 21 through either a transmission line in operation (hereafter referred to as active transmission line) or a transmission line not in operation (hereafter referred to as standby transmission line. In the example shown in FIG. 1, Ethernet transmission line 5a is the active line.) The switchover of the transmission line caused by a failure is carried out either by a switchover command included in a control frame or by control frame timeout.
In FIG. 2, there is shown a sequence flow chart in a conventional client server system. The figure illustrates a general procedure performed by software both in CPU 31 of client system 1 and in CPU 21 of server system 2.
In client system 1, line controllers 34a, 34b read out the contents of a non-illustrated fault register provided in line controllers 34a, 34b (procedure P1), and determine whether there is a failure on an active line, and whether there is a failure on a standby line for switchover (procedure P2).
In the case there is a failure in working Ethernet transmission line 5a, and there is no failure in standby Ethernet transmission line for switchover 5b (procedure P2; Yes), a transmission line switchover request is sent to server system 2 through Ethernet transmission line 5b using a control frame (procedure P3). The frame format conforms to IEEE 802.3 standard established by the IEEE 802 Committee, or the like.
In server system 2, a control frame received by line controller 24b is analyzed (procedure P4).
Through this control frame analysis, if the line switchover request is recognized and there is no failure on the standby line for switchover (procedure P5; Yes), a response to the line switchover request is sent to client system 1 (procedure P6).
On receiving the response of the line switchover request from server system 2, client system 1 analyzes a received control frame (procedure P7). If a received response corresponds to the line switchover request and there is no failure on the standby line for switchover (procedure P8), client system 1 send a request for performing the switchover (procedure P9).
Server system 2 analyzes the received control frame (procedure P10), and recognizes the request for performing the switchover and there is no failure on the standby line for switchover (procedure P11; Yes), and performs the switchover the operation condition of the transmission line (procedure P12).
Server system 2 then informs client system 1 of the completion of switchover (procedure P13). Client system 1 analyzes the received frame (procedure P14) and recognizes the response of the switchover. If there is no failure on the line to be switched over (procedure P15; Yes), client system 1 switches the active transmission line to be consistent with server system 2 (procedure P16).
Thus, in a conventional system, the switchover of the active line to standby and vise versa is performed by transmitting and receiving a control frame between client system 1 and server system 2, using a line having no failure at the time of switchover.
There are the following problems in such a conventional system. First, as mentioned above, both data frames directed to input/output controller 36 and control frames directed to CPU 31 are controlled by the identical Ethernet controller 33a (33b) in client system 1 shown in FIG. 1.
CPU 31 distinguishes control frames from data frames by determining control information in each frame higher than the MAC layer. Therefore, when there are large data frame traffic directed to input/output controller 36, delay may arise in processing control frames in CPU 1. When the situation is worse, this may possibly be misidentified as communication failure.
Another problem in the conventional system is that when performing the switchover control, a complicated sequence processing is required in CPU 31, as shown in FIG. 2. Further, because the conventional switchover control requires information transmission and reception between the server and the client, and in some cases the switchover control is started after the timeout of the control frame, it takes a substantial time for switchover procedure.